The majority of syringes used today for medical or laboratory purposes are disposable and are intended to be discarded after a single use. Disposal of the syringes poses a potential hazard for the individuals who use the syringes, as well as for those who dispose of them. With the increased awareness concerning the potential for the transmission of AIDS and other infectious diseases, a number of different devices have been proposed to prevent accidental needle sticks or to otherwise disable the syringe after a single use.
One approach to this problem is to provide a protective shield which is movable between a retracted position and an extended position. A common feature of these syringes is that when the retracted shield is moved to the extended position, the shield covers the needle and cannot be retracted without the use of extraordinary force.
A number of such constructions have been proposed to satisfy the general requirement that the needle be permanently covered after the syringe has been used. Some of these constructions involve twist-to-lock mechanisms which often require at least two hands to operate and do not readily indicate when the shield has been locked into the extended position. Other syringes lock automatically when the shield is extended. In many of these syringes, the locking mechanism remains exposed and may be manually manipulated to retract the shield after the shield has been "irreversibly" locked.
Although many of these constructions meet the general requirements for a safety syringe, many of these devices are much too complicated to manufacture economically and efficiently. Certain of these devices require extensive modification of the conventional syringe components. This is generally unacceptable due to the large investment most syringe manufacturers have in their present molding and extrusion equipment. Other devices cannot realistically be mass produced using presently known manufacturing techniques. Finally, certain other devices require a large number of manipulative steps during the assembly process and thus cannot be efficiently mass produced.
Finally, many of the proposed safety syringes require the health care worker to use both hands to perform complex manipulations of the safety syringe in order to move the protective shield to the extended needle protecting position. Typically, the user must hold the syringe barrel with one hand while the other hand is used to move the protective shield distally to the extended position. In certain other constructions, the user must place their hands unacceptably close to the potentially infective needle to move the protective shield to the extended position. In many situations, it is preferable for the user to be able to move the protective shield to an extended and locked position by safely moving their thumb distally along the barrel section of the syringe.